Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device with reduced operating voltage, improved current efficiency, and improved lifetime characteristics.
Discussion of the Related Art
Image display devices used for displaying a variety of information on the screen are one of the core technologies of the information and communication era. Such image display devices have been developed to be thinner, lighter, and more portable, and furthermore to have high performance. With the development of the information society, various demands for display devices are increasing. To meet these demands, research on flat panel displays such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), field emission displays (FED), organic light emitting diodes (OLED), etc is actively under way.
Among the panel displays, the OLED devices are a type of devices that emit light as electrons and holes are paired and extinguished, when a charge is injected into an organic light emitting layer between an anode and a cathode. The OLED devices are advantageous in that they may be formed on a flexible transparent substrate such as plastic, and are operable at relatively low voltage as compared to plasma display panels or inorganic EL displays, less power consumption, and represent excellent color sensitivity. Especially, white OLED devices are used for various purposes in lighting, thin light sources, backlights for liquid crystal displays, or full-color displays employing color filters.
In the development of white OLED devices, high efficiency, long lifetime, color purity, color stability against current and voltage variations, ease of manufacture, etc are important, so research and development are being done depending on which of these features should be taken into account. White OLED device structures may be roughly classified into a single-layer emitting structure and a multilayer emitting structure. Of these structures, the multilayer emitting structure having a blue fluorescent emitting layer and a yellow phosphorescent emitting layer stacked in tandem is mainly employed to realize white OLED devices with long lifetime.
Specifically, a phosphorescent emitting structure is used, which is a lamination of a first light emitting part using a blue fluorescent diode as a light emitting layer and a second light emitting part using a yellow-green phosphorescent diode as a light emitting layer. Such a white OLED device produces white light by mixing blue light emitted from the blue fluorescent diode and yellow light emitted from the yellow-green phosphorescent diode. In this case, a charge generation layer is formed between the first light emitting part and the second light emitting part to double the current efficiency generated in the light emitting layers and facilitate charge distribution. The charge generation layer is a layer that generates a charge, i.e., electrons and holes, in it, which can prevent a rise in operating voltage because it doubles the current efficiency generated in the light emitting layers and facilitates charge distribution.
However, the overall operating voltage of a device having the aforementioned multilayer emitting structure may be higher than the sum of the operating voltages of individual light emitting parts, or the multilayer emitting device may suffer a decrease in efficiency compared to a single-layer emitting structure. The charge generation layer includes an N-type charge generation layer and a P-type charge generation layer. Especially, if the N-type charge generation layer is doped with an alkali metal or alkali earth metal, the alkali metal or alkali earth metal may be diffused into the P-type charge generation layer, causing a decrease in device lifetime. Also, the difference in LUMO (lowest unoccupied molecular orbital) energy level between the P-type charge generation layer and the N-type charge generation layer deteriorates the feature of injecting electrons generated at the interface between the P-type charge generation layer and the hole transport layer into the N-type charge generation layer. Moreover, the difference in LUMO energy level between the electron transport layer and the N-type charge generation layer causes a rise in operating voltage when the electrons injected into the N-type charge generation layer move to the electron transport layer.